A fired metal paste has been conventionally used as electrodes constituting gas sensor electrodes and heater electrodes of various kinds of gas sensors such as an oxygen sensor, an NOx sensor, and an exhaust temperature sensor. The reason why a metal paste is applied to production of these electrodes is because the metal paste can cope with complicated electrode patterns, and in addition, a substrate and an electrode can be produced simultaneously by applying and firing a metal paste on a green sheet forming a ceramic substrate, and this is preferable from the perspective of the production efficiency.
A known configuration of a metal paste for forming an electrode includes a mixture obtained by mixing conductive particles such as precious metal and ceramic powder such as Al2O3 or ZrO2 into a solvent. The reason why ceramic powder is mixed into the metal paste is because, when a substrate and an electrode are produced at a time by applying and firing the metal paste to the green sheet as described above, a difference in the shrinkage rate between the metal paste and the green sheet is corrected, the problem of warping and deformation of the substrate is solved, and the adhering property of the electrode is improved. In fact, the ceramic powder ensures the formation property of an electrode film, but on the other hand, there is a disadvantage in that the resistance value of the produced electrode film is increased to a higher level than an electrode of a bulk metal. For this reason, when the ceramic powder is used, it is a problem to study the optimum usage mode and mixing amounts while achieving the balance between ensuring of the formation property and the resistance reduction of the electrode.
In association with the above problem, the present inventors disclose a metal paste that can produce a low resistance electrode film and has superior adhering property and adapting property to a substrate and an electrode produced thereby (Patent Document 1). In the metal paste according to the present inventors, a core/shell structure obtained by combining ceramic particles with the external surface of a core particle including precious metal to cover the external surface with the ceramic particles is applied to the configuration of the conductive particles. With the conductive particle having the core/shell structure, ceramic particles are dispersed finely during the step of firing the metal paste, in order to suppress the increase in the size of the ceramic powder that increases the resistance. As a result, the fired electrode becomes closely packed and the resistance thereof is low.